Rapid advances in electronics and communication technologies, driven by immense customer demand, have resulted in the widespread adoption of mobile communication devices. The extent of the proliferation of such devices is readily apparent in view of some estimates that put the number of wireless subscriber connections in use around the world at nearly 80% of the world's population. Furthermore, other estimates indicate that (as just three examples) the United States, Italy, and the UK have more mobile phones in use in each country than there are people living in those countries.
Mobile communication devices and other radio devices communicate over radio channels though a transmission medium such as air. To accurately and reliably receive a communicated radio transmission, a receiver develops a channel estimate or channel state information. The channel estimate includes at least magnitude and phase information. More generally, the channel state information describes how a signal propagates from transmitter to the receiver including effects of scattering, fading and power decay with distance. Using the channel estimate or channel state information, the receiver can adapt to current and changing conditions.